1. Field of the Invention
The present invention relates to a method of transmitting non-orthogonal physical channels by a single transmitter in the communications system.
2. Description of the Related Art
In a multiple access mobile communications system, a plurality of physical channels may be transmitted through the same frequency band at the same time. In this case, interference may occur among the physical channels to deteriorate the quality of communications.
In order to prevent the deterioration of the communication quality as above, a method of allocating inherent channelization codes to the respective physical channels is used in a code division multiple access (CDMA) mobile communications system.
In other words, the frequency band of data transmitted through the respective physical channel is spread by multiplying the data by an inherent channelization code allocated to the corresponding physical channel. It means that transmission of the data multiplied by its channelization code (i.e., after being spread) occupies much more frequency bands than transmission of the data as it is since a chip rate, that is a transmission rate of the channelization code, is higher than a bit rate, that is a transmission rate of the data.
Here, a value obtained by dividing the chip rate by the bit rate is called a spreading factor. This spreading factor means the number of the channelization code chips multiplied by a single data bit.
When receiving the data, a receiver multiplies a received signal by an inherent channelization code allocated to a corresponding physical channel to be received, and then integrates the multiplied value for a bit period. That is, the receiver already knows the inherent channelization code of the physical channel that the receiver intends to know.
The channelization code of the physical channel that the receiver intends to know is composed of chips having the value of “−1” or “1”.
As described above, if the value of the chip is twice multiplied by the same channelization code, the value always becomes “1”, so that the influence due to the channelization code vanishes, and only the data signal component remains in the signal.
Meanwhile, the signal component due to another physical code corresponding to the interference is multiplied by the channelization code in the transmitter, and then is multiplied by another channelization code in the receiver, so that the resultant signal is in the form of a noise because the channelization code is not removed, but the strength of the noise signal is greatly reduced as it passes through an integrator in the receiver.
Especially, in the downlink of the CDMA system, the physical channels are discriminated using an orthogonal variable spreading factor (OVSF) codes which are the mutually orthogonal channelization codes. The OVSF code serves to make the physical channel signals mutually orthogonal after spreading irrespective of data values among timing-synchronous physical channels or the spreading rate of the physical channels.
In case that the physical channels are mutually orthogonal, no interference occurs among the signals transmitted from the same transmitter through the same path, and a large number of physical channels can be transmitted without deterioration of the communication quality.
In a wideband CDMA (W-CDMA) system proposed to support an IMT-2000 service, a scrambling code is used for discriminating base stations or cells. This scrambling code is a code allocated for each base station or cell.
It is assumed that the scrambling code is allocated for each base station. If the number of the physical channels per base station is smaller than the number of the usable OVSF codes, it is preferable that one scrambling code is used for one base station. However, If the number of the physical channels per base station is larger than the number of the usable OVSF codes, a plurality of scrambling codes should be used. In case of using a plurality of scrambling codes, an orthogonality is effected among the physical channels using the same scrambling code due to the OVSF code, but no orthogonality is effected among the physical channels using different scrambling codes.
A first used scrambling code is named a primary scrambling code, and an additionally used scrambling code is named a secondary scrambling code. A plurality of secondary scrambling codes exists in a base station, and it is assumed that the number thereof is “M”.
FIG. 1 is a block diagram illustrating the construction of a spreader in a W-CDMA base station transmitter which is a conventional radio connection device for supporting the IMT-2000 service, and FIG. 2 is a block diagram illustrating the construction of a modulator in the W-CDMA base station transmitter.
Referring to FIGS. 1 and 2, as codes used in the W-CDMA base station transmitter exist the channelization code and the scrambling code (based on the bibliography; 3GPP RAN 25.213, v2.1.0 (1999-04) spreading and Modulation (FDD)).
The channelization code is inherently allocated for the physical channel, and the base station uses one primary scrambling code and M secondary scrambling code.
First, a spreading process will be explained.
The physical channel is composed of a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH). The signal of the physical channel is divided into an I-channel branch signal and a Q-channel branch signal by a serial-to-parallel converter, and the I-channel and Q-channel branch signals are multiplied by the OVSF code of the corresponding physical channel through mixers to be spread.
The spread I-channel branch signals and the spread Q-channel signals are summed by an I-channel branch signal summer and a Q-channel branch signal summer, respectively. The summed Q-channel branch signal is converted into an imaginary number through an imaginary number converter, and then the converted imaginary number and the summed I-channel branch signal are combined into a complex number through a complex combiner. This complex number is complex-valued-scrambled by a specified complex-valued scrambling code in a mixer.
Next, a modulation process will be explained with reference to FIG. 2.
The signals complex-valued-scrambled by the scrambling codes whose number is M+1 are summed together by a complex summer, and the summed signal is divided into a real part and an imaginary part. The real part signal and the imaginary part signal pass through pulse modulation function sections, and are multiplied by cosωt and −sinωt, respectively. Finally, the multiplied signals are summed by a summer to be transmitted through an antenna.
As shown in FIGS. 1 and 2, when the base station transmits the signals, a timing synchronization is effected among the physical channels, and there exists no difference among starting points of chip transmission for the respective physical channels. FIG. 3 shows the difference among starting points of chip signals of the respective physical channels in the conventional W-CDMA base station transmitter.
FIG. 4 is a block diagram illustrating the construction of a demodulator in the conventional W-CDMA base station receiver. The demodulation process performed thereby is as follows.
The signal received through a mobile station receiver antenna is divided into two signals, and the divided signals are multiplied by cosωt and sinωt, respectively. The multiplied signals are converted into digital signals, and the digital signals pass through a same chip-matched filter. Then, the signal component multiplied by sinωt passes through an imaginary number converter to be converted into an imaginary number. The imaginary number outputted from the imaginary number converter and the signal component multiplied by cosωt are combined into a complex number through a complex combiner. The complex number outputted from the complex combiner, which is a signal in the unit of a sample, is converted into a signal in the unit of a chip through an under-sampling block. The signal outputted from the under-sampling block is used as an input of despreaders of all scrambling codes. Specifically, since the W-CDMA base station transmits the physical channels which have no difference of starting points of chip transmission irrespective of the scrambling codes (i.e., existence/nonexistence of mutual orthogonality) used by all the physical channels, it is not required for the W-CDMA mobile station receiver to separately match the starting points of chip transmission for respective physical channels.
As described above, in case that the physical channels are synchronous in timing, the orthogonality can be effected among the physical channels having the same scrambling code due to the property of the OVSF code.
In order to synchronize the physical channels in timing, the starting points of chip transmission of the physical channels should be identical. However, the orthogonality cannot be effected among the physical channels having different scrambling codes even if they are synchronous in timing (i.e., they have the same starting point of chip transmission). Further, in case that the physical channels have the same starting point of chip transmission as in the related art, even the physical channels having no orthogonality have the same starting point of chip transmission, and this causes the interference among the physical channels having no orthogonality to become greatest (M. B. Pursley, “Performance evaluation of phase-coded spread-spectrum multiple-access communication-part I: system analysis,” IEEE Trans. Commun. Vol. COM-25, no. 8, August 1977, pp. 795–799).
There also exists the problem that the interference among the physical channels having no orthogonality becomes great in a cdma2000 system having the standard different from that of the W-CDMA system and supporting the IMT-2000 service. In the downlink of the cdma2000 spread spectrum system for supporting the IMT-2000 service, the physical channels are discriminated using a Walsh function which corresponds to channelization codes orthogonal with one another.
This Walsh function serves to make the physical channel signals mutually orthogonal after spreading irrespective of data values among timing-synchronous physical channels or the spreading rate of the physical channels.
In case that the physical channels are mutually orthogonal, no interference occurs among the signals transmitted from the same transmitter through the same path, and a large number of physical channels can be transmitted without deterioration of the communication quality.
Also, in the cdma2000 spread spectrum system, a quasi-orthogonal function (QOF) is used in addition to the Walsh function. The Walsh function is used if the number of physical channels per cell is smaller than the number of usable Walsh functions, while the quasi-orthogonal function (QOF) is used if the number of physical channels per cell is larger than the number of usable Walsh function.
Three quasi-orthogonal functions (QOFs) are defined (Refer to 3GPP2 C. S0002-A, Physical Layer Standard for cdma 2000 Spread Spectrum Systems, Release A.). Here, if the Walsh function it self is considered as a quasi-orthogonal function, the number of the quasi-orthogonal functions becomes four, and hereinafter, it is considered that the number of the quasi-orthogonal functions is four.
In the event that a plurality of quasi-orthogonal functions are used as the channelization codes in a cell, the orthogonality is effected among the equal quasi-orthogonal functions, and no interference occurs among the physical channels using the equal quasi-orthogonal functions as their channelization codes. However, no orthogonality is effected among the different quasi-orthogonal functions, and this causes the interference to occur among the physical channels using the different quasi-orthogonal functions.
FIG. 5 is a block diagram of a transmitting device of the conventional cdma2000 system. In FIG. 5, a transmitting device of a base station in the conventional cdma2000 system is illustrated.
Referring to FIG. 5, the transmitting device comprises a first mixer 100 for generating a channelization code by multiplying a Walsh function by a sign of a quasi-orthogonal function, spreaders 110 and 111 for spreading an input I-channel signal and Q-channel signal by multiplying them by the channelization code, respectively, a rotator 120 for rotating the spread I-channel signal and Q-channel signal on an I plane and a Q plane, respectively, in accordance with a Walsh rotation value, a complex multiplier 130 for complex-multiplying the I-channel signal and the Q-channel signal outputted from the rotator 120 by multiplying the I-channel signal and the Q-channel signal by a pseudo noise code, and a modulator 150 for modulating the I-channel signal and the O-channel signal outputted from the complex multiplier 130 by multiplying the I-channel signal and the Q-channel signal by a carrier.
Here, the spreader 110 comprises a second mixer 111 for spreading the I-channel signal by multiplying the I-channel signal by the generated channelization code, and a third mixer 112 for spreading the Q-channel signal by multiplying the Q-channel signal by the generated channelization code.
The complex multiplier 130 comprises a fourth mixer 131 for multiplying the I-channel signal outputted from the rotator 120 by the pseudo noise code of the I channel, a fifth mixer 132 for multiplying the I-channel signal outputted from the rotator 120 by the pseudo noise code of the Q channel, a sixth mixer 133 for multiplying the Q-channel signal outputted from the rotator 120 by the pseudo noise code of the I channel, a seventh mixer 134 for multiplying the Q-channel signal outputted from the rotator 120 by the pseudo noise code of the I channel, a first combiner 135 for summing an output signal of the fourth mixer 131 and an output signal of the sixth mixer 133, and a second combiner 136 for summing an output signal of the fifth mixer 132 and an output signal of the seventh mixer 134.
The operation of the transmitting device of the conventional communication system as constructed above will new be explained.
The respective physical channel is composed of an I-channel branch signal XI and a Q-channel branch signal XQ. Then, the I-channel branch signal and the Q-channel branch signal are spread by being multiplied by the channelization codes of the corresponding physical channels, respectively, as they pass through the mixers 111 and 112.
The respective channelization code is generated by multiplying the Walsh function by the sign of the quasi-orthogonal function (QOF) QOFsign. The I-channel branch signal Iin and the Q-channel branch signal Qin rotate by 0° or 90° on the I and Q planes by the Walsh rotation value Walshrot in the rotator 120.
Thereafter, the I-channel branch signal and the Q-channel branch signal are complex-multiplied by a pseudo random code PNI+jPNQ in the complex multiplier 130. After the complex multiplication, a real output value I and an imaginary output value Q pass through baseband filters 140 and 141, respectively, and then modulated to a carrier frequency in the modulator 150.
Here, the Walsh sign QOFsign, and the Walsh rotation value Walshrot for generating the quasi-orthogonal function QOF are presented in the following Table 1.
TABLE 1Index (QOF)Hexadecinal expression of QOFsignWnN (Walshrot)0000000000000000000000000000000W025600000000000000000000000000000017228d7724eebebb1eb4eb1ebd78d8d28W130256278282d81b41be1b411b1bbe7dd8277d2114b1e4444e1ebeeee4de144bbe1b4eeW173256dd872d77882d78dd2287d277772d87dd31724bd71b28118d48ebddb172b187ebe2W47256e7d4b27ebd8ee82481b88be7dbe871bd
In Table 1, WnN means a value the length of which is N, and the Walsh code index of which is n. The timing synchronization is effected among all the physical channels transmitted from the base station, and thus there exists no difference of starting points of chip transmission among the physical channels.
The difference of starting points of chip transmission among the respective physical channels in the cdma2000 base station transmitting device is shown in FIG. 6.
Referring to FIG. 6, it can be recognized that the starting points of chip transmission of the respective physical channels are identical. FIG. 7 is a block diagram of a receiving device of a conventional cdma2000 system. In FIG. 7, the receiving device of a mobile station in the conventional cdma2000 system is illustrated.
Referring to FIG. 7, the receiving device of the mobile station comprises a demodulator 300 for demodulating a received signal to a baseband signal by multiplying the received signal by a sine carrier and a cosine carrier, respectively, analog-to-digital converters 303 and 304 for converting demodulated signals onto digital signals, baseband filters 305 and 306 for filtering the respective digital signals outputted from the analog-to-digital converters 305 and 306, and under-sampling blocks 307 and 308 for converting filtered digital signals in the unit of a sample into signals in the unit of a chip, and transmitting the converted signals to a pseudo noise decoder. The numeral “L” denotes the number of samples per chip in the under-sampling blocks 307 and 308.
The operation of the receiving device of the mobile station as constructed above will now be explained.
The signal inputted through a receiving antenna of the mobile station is divided into two signals. The two signals are demodulated into the baseband signals by the demodulators 301 and 302, and then converted into the digital signals by the analog-to-digital converters 303 and 304.
The converted digital signals pass through the baseband filters 305 and 306, converted into the signals in the unit of a chip through the under-sampling blocks 307 and 308, and then transmitted to the pseudo noise code decoder (not illustrated). As described above, in the base station of the cdma2000 system, all the physical channels are transmitted without the difference of starting points of chip transmission irrespective of the index of the quasi-orthogonal function (i.e., existence/nonexistence of the mutual orthogonality) used by the physical channels, it is not required for the receiver of the mobile station of the cdma2000 system to separately match the starting points of chip transmission for the respective physical channels.
As described above, in the cdma2000 system, in case that the physical channels having the same quasi-orthogonal function (QOF) are synchronous in timing due to the property of the Walsh function. Accordingly, in order to synchronize the physical channels in timing, the starting points of chip transmission of the physical channels should be identical.
However, the orthogonality cannot be effected among the physical channels having different quasi-orthogonal functions (QOF) even if they are synchronous in timing (i.e., they have the same starting point of chip transmission).
Further, in case that the physical channels have the same starting point of chip transmission as in the related art, even the physical channels having no orthogonality have the same starting point of chip transmission, and this causes the interference among the physical channels having no orthogonality to increase.